Hydrostatic Piston Machine

ABSTRACT

The invention relates to a hydrostatic piston machine ( 1 ) having a drive shaft ( 2 ) which passes through a cylinder-drum unit. Cylindrical clearances ( 18, 19 ) are disposed in said cylinder-drum unit. Pistons ( 16, 17 ), which are disposed in a displaceable manner in said cylinder clearances ( 18, 19 ), are connected to the drive shaft ( 2 ) in a torsion-proof manner. The cylinder-drum unit is centred on a bearing ( 32, 32 ′) provided on the drive shaft ( 2 ). Said cylinder-drum unit consists of a cylinder drum ( 20, 21 ) having a number of cylindrical clearances ( 18, 19 ) constructed therein.

The invention relates to a hydrostatic piston machine with a cylinder-drum unit which has a drive shaft passing through it, pistons being disposed on said drive shaft in a torsion-proof manner.

A hydrostatic piston machine in which a drive shaft passes through a first and a second cylinder-drum unit is known from PCT/NL03/00017. A carrier plate is fixed on the drive shaft symmetrically between the two cylinder-drum units. Pistons which are displaceably mounted in cylinder spaces in the cylinder-drum units are disposed opposite one another on the carrier plate. The axis of rotation of each of said cylinder-drum units is at an angle with respect to the angle of the drive shaft. In this way, the pistons, which are connected to the drive shaft in a torsion-proof manner, execute a stroke-type movement within the cylinder spaces. Said cylinder spaces are constructed in individual cylinders which are supported on a common drum plate and form a cylinder unit with the latter. In order to prevent the cylinders from lifting off the drum plate, a retaining device, which permits a radial movement of the cylinders, is constructed. This arrangement, which is universally displaceable, permits a compensating movement of the cylinders which is necessary because of the fixed arrangement of pistons in the carrier plate and the angle of inclination of the cylinder-drum unit in relation to the axis of the drive shaft.

The arrangement described has the disadvantage, especially at high rotational speeds, that the centrifugal forces which are operating lead to a pitching moment on the part of the cylinders. Although the cylinders are individually held in contact with the drum plate via a retaining device, a considerable degree of wear nevertheless occurs because of the large number of parts which move, relative to one another, since there is only a small bearing face on the cylinders for the forces which are operating.

It is the object of the invention to provide a hydrostatic piston machine in which the aforesaid disadvantages, which come about as a result of the large number of interacting components of the cylinder-drum unit, are avoided.

In the hydrostatic piston machine according to the invention having the features according to claim 1, the cylinder-drum unit likewise has a drive shaft passing through it. Pistons which penetrate into the cylindrical clearances in the cylinder-drum unit are connected to the drive shaft in a torsion-proof manner. In contrast to the prior art, the cylindrical clearances are disposed jointly in a cylinder drum which forms the cylinder-drum unit. This cylinder drum, which is thereby designed in a compact form, is centred, as a whole, on a bearing provided on the drive shaft. The individual cylindrical clearances in the cylinder drum thus do not execute any independent movements. On the contrary, centring of the entire cylinder drum, relative to the axis of rotation of the drive shaft, is achieved. As a result of the one-piece design of the cylinder drum itself, the individual cylindrical clearances likewise cannot execute any pitching movement when centrifugal forces occur.

Because the pitching movement of the individual cylinders is prevented, the wear that sets in will be reduced. Also avoided, in particular, is canting in the direction of rotation, such as can occur in the individual cylinders when the rotating movement of the cylinder-drum unit is brought about by the pistons disposed in the cylinders.

Advantageous further developments of the hydrostatic piston machine according to the invention are represented in the subclaims. It is of particular advantage to connect the pistons to the drive shaft via an articulated connection. The torsion-proof connection of the pistons to the drive shaft via an articulated connection has the advantage that omnidirectional compensation does not have to take place by displacement of the cylinders. On the contrary, the compensating movement is achieved by the pistons having a different inclination, relative to the axis of the drive shaft, this being made possible by the articulated connection.

It is also advantageous to provide the piston, for this purpose, with a spherical head which engages in a corresponding spherical clearance in the drive shaft, or in a carrier plate connected to said drive shaft. In this case, the carrier plate may be designed as a radial widened portion on the drive shaft itself, for example as a one-piece forging. It is equally conceivable to provide the drive shaft with a toothing system which is in engagement with a separately manufactured carrier plate, and thus forms a torsion-proof connection to the latter. As a result of the construction of the articulated connection between the piston and the drive shaft, inclination of the axis of the piston with respect to the axis of the drive shaft is possible. At the same time, a direct, torsion-proof connection between the drive shaft and the piston is achieved as a result of the pistons being fixed, in the form of a ball joint, on the carrier plate.

In order to prevent rotation being blocked by contact of the piston shaft against the inner wall of the cylinder when the pistons penetrate into the cylindrical clearances, a conical section is constructed between a sealing section and the articulated connection on the pistons. The angle of opening of the conical section preferably corresponds to the intended maximum angle of inclination of the cylinder drum.

Centring of said cylinder drum is preferably achieved by means of a bearing which is constructed on the drive shaft. For this purpose, a central through-aperture in the cylinder drum, which is preferably designed as a bore, interacts in a centring manner with a bearing disposed on the drive shaft. Under these circumstances, the bearing is preferably constructed, on the drive shaft side, as a spherical drive-shaft section.

According to one simple form of embodiment, the torque between the cylinder drum and the drive shaft can come about through the pistons disposed in the cylindrical clearances. According to one preferred form of embodiment, however, an entraining element is provided between the spherical drive-shaft section and the cylinder drum. The cylinder drum and the spherical section of the drive shaft are connected to one another in a torsion-proof manner by the said entraining element. Under these circumstances, it is particularly preferred if the entraining element is fixed in the drive shaft and engages in an entraining groove in the cylinder drum. The construction of an entraining groove in the through-aperture in the cylinder drum guarantees that the angle of inclination of the axis of said cylinder drum can be varied relative to the axis of rotation of the drive shaft.

Instead of disposing the entraining element in the region of the spherical drive-shaft section, it is also conceivably possible to dispose it in a fixed manner in the cylinder drum. An entraining groove is then accordingly disposed in said spherical drive-shaft section. It is also possible to provide a number of entraining elements which are preferably distributed uniformly over the periphery.

In order to obtain the best possible sealing action, piston rings are preferably disposed on the pistons in a sealing section of the latter. Said pistons rings are designed, for example, as steel rings which are inserted in a corresponding groove on the side of the pistons in the region of the sealing section. The piston rings are preferably likewise of spherical design on their outer contour.

Another possible way of improving the sealing action between the pistons and the corresponding cylindrical clearance is to construct a thin-walled piston shaft in the region of the sealing section. A thin-walled piston shaft of this kind in the region of the sealing section enables the pressure prevailing in the interior of the cylinder to expand said piston shaft elastically, as a result of which its outer contour is applied against the cylindrical clearance in a sealing manner. This can be carried out in a particularly simple way by means of a clearance in the piston, which clearance is located on the side that faces away from the articulated connection.

In order to prevent the pistons from lifting off the carrier plate during the intake stroke, a retraction disc which fixes the pistons on said carrier plate in the axial direction is preferably provided. Under these circumstances, said retraction disc is disposed in such a way that the inclination of the pistons relative to the carrier plate is not hindered. Said retraction disc encloses the pistons in such a way that it acts as a constituent part of the articulated connection. For this purpose, clearances in the retraction disc are provided which accommodate the head of the pistons and have a spherical contour.

A preferred exemplified embodiment of the hydrostatic piston machine according to the invention is represented in the drawings and will be represented in a detailed manner in the following description. In said drawings:

FIG. 1 shows a longitudinal section through a first exemplified embodiment of a hydrostatic piston machine according to the invention;

FIG. 2 shows a longitudinal section through a second exemplified embodiment of a hydrostatic piston machine according to the invention; and

FIG. 3 an enlarged representation of the part III of FIG. 2.

A longitudinal section through a first exemplified embodiment of a hydrostatic piston machine 1 according to the invention is represented in FIG. 1. Said hydrostatic piston machine 1 has a drive shaft 2 which is mounted in a housing formed from a first half 3 and a second half 4. The first half 3 of the housing and the second half 4 of said housing are of approximately pot-shaped construction. Under these circumstances, a through-aperture 5 is constructed in the first half 3 of the housing. In the exemplified embodiment represented, said through-aperture 5 is designed as a stepped bore. An end of the drive shaft 2 which is provided with a toothing system 6 protrudes through said through-aperture 5.

A first drive-shaft bearing 7 is disposed in the stepped through-aperture 5. Said first drive-shaft bearing 7 is designed as a tapered-roller bearing. Also provided in the stepped through-aperture 5 is a sealing element which seals the drive shaft 2 in relation to the first part 3 of the housing.

At the opposite end of the housing, a stepped blind bore 8 is constructed in the second part 4 of said housing. A second drive-shaft bearing 9, which is likewise constructed as a tapered-roller bearing, is disposed in that part of said stepped blind bore 8 which is oriented towards the interior space of the housing.

The first part 3 and the second part 4 of the housing each have a circumferential flange, 10 and 11 respectively. The first part 3 and the second part 4 of the housing are screwed to one another at the flanges 10, 11 with the aid of screws 12.

A carrier plate 13 is disposed, approximately centrally in the interior of the housing, on the drive shaft 2. In the exemplified embodiment represented, said carrier plate 13 is designed in one piece with said drive shaft 2. Spherical clearances 14, 15 are incorporated, in the form of joint sockets, in the carrier plate 13 on both sides. Pistons 16, 17 having ball-shaped heads disposed at their ends are inserted in said spherical clearances 14, 15. The articulated connection thus produced between the pistons and the carrier plate 13 will be explained again in detail below with reference to FIG. 3. The pistons 16 and 17 are of identical design. For the sake of clarity, the reference numerals 16 and 17 each designate just one piston of the group of pistons which interact with a cylinder drum 20, 21.

The pistons 16, 17 protrude, with their end that faces away from the articulated connection between the pistons 16, 17 and the carrier plate 13, into cylindrical clearances 18, 19 in a first cylinder drum 20 and a second cylinder drum 21, respectively. Within said cylinder drums 20, 21, a plurality of cylindrical clearances, 18 and 19 respectively, are disposed, parallel to one another, over a first and a second peripheral circle, respectively. The common peripheral circle of the first cylinder drum 20 and the common peripheral circle of the second cylinder drum 21 are preferably identical and correspond to the peripheral circles on which the spherical clearances 14, 15 on the carrier plate 13 are disposed.

A cylindrical volume is enclosed, in each case, between the cylindrical clearances, 18 and 19 respectively, on one common peripheral circle each, and the corresponding pistons (16, 17) disposed therein. The cylinder drums 20, 21 are disposed in an inclined manner in relation to the axis of rotation of the drive shaft 2. When a rotation of the cylinder drums 20, 21 and also of the drive shaft 2 occurs, the pistons, 16 and 17 respectively, execute a stroke-type movement in the corresponding cylindrical clearances, 18 and 19 respectively, and thereby diminish and enlarge the enclosed cylindrical volume cyclically. The cylinder drums 20 and 21 are each supported on a slanting disc, 22 and 23 respectively. In the exemplified embodiment represented, said slanting discs 22, 23 are disposed in a fixed manner, so that a constant angle of inclination of the first and second cylinder drums 20, 21 in relation to the axis of the drive shaft 2 is set up. However, it is equally possible to support the cylinder drums 20, 21 against one adjustable slanting disc each and to thus design the stroke volume in a settable manner. In this case, it is possible, in particular, to provide independent adjustment of the inclinations of the first cylinder drum and second cylinder drum, 20 and 21 respectively. Another possibility is to provide one of the cylinder drums, 20 or 21, with a fixedly set angle and to provide the other cylinder drum 20, 21, in each case, with an angle of pivoting which can be changed.

The exemplified embodiments represented relate, in each case, to double pumps or double motors. In the figures, features which are explained only for one side are provided, on the opposite side, with a corresponding reference numeral having an apostrophe.

The following embodiments relate to that unit of the machine which is represented on the left-hand side in FIG. 1 and consists of the slanting disc 22, the first cylinder drum 20 and the pistons 16 which execute a stroke-type movement therein. The embodiments relate, in an analogous manner, to the second unit of the machine, which is represented on the right in FIG. 1. The structural elements which correspond in each case are disposed symmetrically with respect to a median plane 26.

The first cylinder drum 20 has a running face 25 by which it is supported against a supporting face 24 of the slanting disc 22. In order to secure said slanting disc 22 against twisting, it is fixed, by means of a locating pin 27, in the first, pot-shaped housing part 3 on the bottom 30 of the latter. When the cylinder drum 20 is rotated, the cylindrical clearances 18 can be connected cyclically, via cylinder apertures 28, to control apertures in the slanting disc 22 which are not represented. In the exemplified embodiment represented, an angle of pivoting which is set in a fixed manner, in each case, for the first cylinder drum 20 and for the second cylinder drum 21, is provided both for the first slanting disc 22 and for the second slanting disc 23. The angle of inclination of the cylinder drums 20, 21 in relation to the axis of the drive shaft 2 is determined by a wedge shape on the slanting discs 22, 23. The slanting disc 22 has a bearing face 29, by which it rests on the bottom 30 of the first part 3 of the housing.

The first cylinder drum 20 has a central through-aperture 31 which is designed as a cylindrical bore. The first cylinder drum 31 is supported against a bearing on the drive shaft 2 by means of said central through-aperture 31. The bearing on the drive shaft 2 is constructed as a spherical contoured section 32 on the outer contour of said drive shaft 2. In the exemplified embodiment represented, the spherical contoured section 32 is itself directly produced by the shaping of the drive shaft 2. As a bearing, use may equally well be made of a superimposed element which has a spherical outer contour. In this case, use may be made of different materials for constructing the bearing and for the drive shaft 2 itself, in order to achieve, for example, improved emergency-running properties in the event of a lack of lubrication.

In order to prevent the cylinder drum 20 from lifting off the slanting disc 22, a spring 33 is provided, which is likewise supported against the bearing on the drive shaft 2 via a supporting body 34. For this purpose, said supporting body 34 has a spherical clearance, in the region of its contact against the bearing on the drive shaft 2, which corresponds with the spherical contoured section 32 of said bearing. The outer diameter of the supporting body 34, which is constructed as a ring, corresponds to the inner diameter of the central through-aperture 31 of the first cylinder drum 20. On the side opposite the supporting body 34, the spring 33 is supported against a Seeger ring 35 which is inserted in a groove in the cylinder drum 20. Said spring 33 generates an axial force in the direction of the axis of the cylinder drum, which force presses the running face 25 of the cylinder drum 20 against the supporting face 24 on the slanting disc 22.

A retraction disc 36 is provided in order to prevent the lifting of the pistons 16 out of the spherical clearances 14 in the carrier plate 13 during an intake stroke. Said retraction disc 36 is, for example, screwed to the carrier plate 13 and fixes the ball-shaped heads 43 of the pistons 16 in the respective spherical clearances 14. For this purpose, the retraction disc 36 has a number of apertures 37 which is identical to the number of pistons 16, which apertures are likewise spherically contoured and correspond with the outer contour of the ball-shaped head 43 of the pistons 16. For this purpose, the piston 16 is also ball-shaped in its region that protrudes beyond the spherical clearance 14.

The pistons 16 have a lubricating-oil bore 38 which extends from the bottom 39 of a piston to an opposed, flattened end 40 on the ball-shaped head 43 of said piston 16. Relief of the hydrostatic load on the piston 16 in the articulated connection is achieved by means of the lubricating-oil bore 38.

The hydrostatic piston machine 1 according to the invention may be employed both as a pump and as a motor. If it is used as a pump, the cylinder drums 20, 21 are driven via the toothing system 6. The carrier plate 13, which is designed, in the exemplified embodiment represented, in one piece with the drive shaft 2, is set in rotation by the rotating movement transmitted to said drive shaft 2 by means of the toothing system 6. The pistons 16, 17, which are connected to the carrier plate 13 in a torsion-proof manner with respect to rotation about the axis of the drive shaft 2, thus likewise execute a rotation about said axis of the drive shaft 2. A torque is transmitted to the first cylinder drum 20 and second cylinder drum 21 via the pistons 16 and 17 respectively. Said cylinder drums 20, 21 thus execute a rotating movement about their axes, which are inclined in relation to the axis of the drive shaft 2. In the process, the cylinder drums 20 and 21 are kept in contact with the slanting discs, 22 and 23 respectively, by the springs 33, 331. Because of the inclination of the axes of rotation of the drive shaft 2 to the cylinder drums 20, 21, the pistons 16, 17 execute stroke-type movements within the corresponding cylindrical clearances 18, 19, it being possible for the pressure medium delivered by the variable cylindrical volume during one revolution to be delivered into the same hydraulic circuit or different hydraulic circuits.

In the exemplified embodiment represented in FIG. 1, the required torque for rotating the cylinder drums 20, 21 is transmitted by the pistons, 16 and 17 respectively, from the carrier plate 13 to said cylinder drums 20, 21. In the process, said pistons 16, 17 execute an inclining movement until a conical section, 45 and 46 respectively, lying between a sealing section, 41 and 42 respectively, on the pistons, 16 and 17 respectively, and the ball-shaped head, 43 and 44 respectively, of said pistons 16, 17, is in contact with the cylindrical clearances, 18 and 19 respectively.

The piston 16 has, adjoining its bottom, a sealing section 41. Said sealing section 41 is of thin-walled construction. In the exemplified embodiment represented, the thin-walled design of the sealing section 41 is achieved by means of a clearance 65 which is incorporated in the piston 16 from the bottom side. The piston 16 is of spherical design in its sealing section 41 at its outer periphery. A spherical outer contour of this kind may come about, for example, through the incorporation of a cylindrical clearance 65 in the bottom of the piston, whereupon the thin-walled wall section 66 is contoured in order to achieve the spherical outer contour.

An alternative to the transmission of the torque between the drive shaft 2 and the cylinder drums 20, 21 is represented in FIG. 2. In FIG. 2, the same reference symbols designate the features which are already known from FIG. 1. In order to prevent unnecessary repetition, a further detailed description of the entire piston machine 1′ will be dispensed with.

In the exemplified embodiment in FIG. 2, entraining elements 50, 51 are provided for transmitting a torque between the drive shaft 2 and the cylinder drums 20, 21. Said entraining elements 50, 51 are of identical design and act in the same way, between the drive shaft 2 and the cylinder drums 20, 21, as a torque-transmitting apparatus. The following remarks are therefore limited to the entraining element 50 represented on the left in FIG. 2. The entraining element 50 has a cylindrical section 52. Said cylindrical section 52 is inserted in a clearance 53. The depth of said clearance 53 is greater than the length of the cylindrical section 52 of the entraining element 50. Said entraining element 50 protrudes radially beyond the spherical contoured section 32 of the bearing of the drive shaft 2, the protruding part being constructed as a radially expanded region 54. An end face 55 of the entraining element 50, which end face is constructed on said expanded region 54, is likewise contoured in a spherical manner. In the direction of the longitudinal axis of the entraining element 50, said end face 55 might be connected to the opposite end of the entraining element 50 by a duct 57. The radially expanded region 54 engages in a groove 56 in the cylinder drum 20.

As will be explained again later with reference to FIG. 3, the duct 57 might, as an alternative, be connected, via a connecting duct 58, to a volume formed in the spherical clearance 14 by the flattened end 40 of the piston 16. In the same way, a duct 60 in the entraining element 51 might be connected, via a connecting duct 59, to the corresponding volume which is enclosed behind the piston 17 in the spherical clearance 15.

A second, alternative form of embodiment for a sealing section 411, 42′ of the pistons 16, 17 is represented in FIG. 3. In the sealing section 42′, the piston 17 is constructed as a solid piston 17′. At its end that protrudes into the cylindrical clearance 19, said solid piston 171 likewise has a spherical outer contour 67. A groove 68 is incorporated in the sealing section 421 of the solid piston 171 at the transition to a conical section 46. A sealing ring 69, which interacts with the wall of the cylindrical clearance 19 in a sealing manner, is inserted in said groove 68. The piston ring 69 is preferably manufactured from steel, for example. At its outer face 70 which interacts with the cylindrical clearance 19, said piston ring 69 is preferably likewise contoured in a spherical manner.

In the hydrostatic piston machine 1, 11 according to the invention, it is advantageous that the cylinder drums 20, 21 are designed in one piece in each case. This drastically reduces the portions of the parts which move, relative to one another, and the high degree of stiffness of the cylinder drums 20, 21 leads to good absorption of the lateral forces which occur, both because of the centrifugal forces and also because of internal pressure within the cylindrical clearances 18, 19. The compensating movement of the pistons 16, 17 which is required because of the ellipsoidal movement of the sealing sections 41, 41′, 42, 42′ of the pistons, 16 and 17 respectively, is brought about in a simple manner by means of an articulated connection between the carrier plate 13 and said pistons 16, 17. For this purpose, the pistons 16, 17 are disposed in the carrier plate 13 by means of a ball-joint-like connection, and are each fixed to said carrier plate by a retraction disc, 36 and 361 respectively, for the purpose of preventing an axial movement during the intake stroke. For its part, said retraction disc 36, 36′ forms, under these circumstances, part of the articulated connection, through the fact that the clearances 37, 37′ in the retraction discs 36, 36′ interact, for their part, with the ball-shaped head, 43 and 44 respectively, of the pistons, 16 and 17 respectively. In order to permit slight inclination of the pistons 16, 17, and thereby rotation of the ball-shaped head, 43 and 44 respectively, within the spherical clearance, 14 and 15 respectively, relief of the hydrostatic load on the articulated connections of the pistons 16, 17 is provided.

As a further alternative, those flat sides of the radially expanded region 54 of the entraining element 50, 51 which engage in the groove 56, 56, in the cylinder drum 20, 21, might be pressure-lubricated.

A volume, which is filled with pressure medium, is formed in each case in the spherical clearances 14, 15 in the carrier plate 13 by flattened ends, 40 and 40′ respectively, on the pistons 16, 17. This pressure medium is fed to the ducts 57, 60 in the entraining elements 50, 51 via the connecting ducts 58, 59. A circumferential supply groove 71 is machined into the entraining elements 50, 51 in each case, at an orifice of the connecting ducts 58, 59. Said circumferential supply groove 71 ensures that the duct section 72 provided in the entraining elements, 50 and 51 respectively, is in communication with the connecting ducts, 58 and 59 respectively. It is possible, for example, to provide, in the entraining elements 50, 51, a number of duct sections 72 which are distributed over the periphery of said entraining elements 50, 51 and which connect the circumferential groove 71 to the duct, 57 and 60 respectively. In this alternative, the ducts 57, 60 in the entraining elements 50, 51 are occluded in relation to the end face 55, 55′. The pressure medium fed to the duct 60 via the connecting duct 59 passes out of the bore 73 in the expanded region 541 of the entraining element 51 and ensures pressure-lubricated contact faces of said entraining element 51 within the groove 56′ in the cylinder drum 21.

The invention is not limited to the exemplified embodiments represented. On the contrary, it is possible, in particular, to combine the individual features of said exemplified embodiments with one another in any desired manner. 

1. Hydrostatic piston machine having a drive shaft which passes through a cylinder-drum unit in which there are disposed cylindrical clearances in which pistons connected to said drive shaft in a torsion-proof manner are disposed, wherein the cylinder-drum unit is centred on a bearing provided on the drive shaft, wherein the cylinder-drum unit is constructed as a cylinder drum having a number of cylindrical clearances constructed therein.
 2. Hydrostatic piston machine according to claim 1, wherein the pistons are connected to the drive shaft via an articulated connection.
 3. Hydrostatic piston machine according to claim 2, wherein the pistons have a spherical head which is disposed in a corresponding spherical clearance in a carrier plate connected to the drive shaft.
 4. Hydrostatic piston machine according to claim 1 one wherein the pistons have a conical section disposed between a sealing section their connection to the drive shaft.
 5. Hydrostatic piston machine according to claim 1 wherein the cylinder drum has a central through-aperture which interacts in a centring manner with the bearing on the drive shaft.
 6. Hydrostatic piston machine according to claim 5, wherein the bearing on the drive shaft is constructed as a spherical drive-shaft section.
 7. Hydrostatic piston machine according to claim 6, wherein there is provided, in the region of the spherical drive-shaft section, at least one entraining element by which the drive shaft and the cylinder drum are connected to one another in a torsion-proof manner.
 8. Hydrostatic piston machine according to claim 7, wherein the entraining element is fixed in the drive shaft and engages in an entraining groove in the cylinder drum.
 9. Hydrostatic piston machine according to claim 7, the entraining element is fixed in the cylinder drum and engages in an entraining groove in the drive shaft.
 10. Hydrostatic piston machine according to claim 1 wherein a sealing section having a piston ring is constructed on the pistons.
 11. Hydrostatic piston machine according to claim 1 wherein the pistons have a spherical outer contour in the region of their sealing section.
 12. Hydrostatic piston machine according to claim 1 wherein in the region of their sealing section, the pistons have a thin-walled piston shaft which can be expanded elastically by the internal pressure in the cylinder.
 13. Hydrostatic piston machine according to claim 1 wherein the pistons are fixed in the axial direction by a retraction disc. 